The present disclosure relates to a fixing device to fix a toner image on a recording medium, such as a sheet or a film, and an image forming apparatus provided with the fixing device.
An electrographic image forming apparatus, such as a copying machine and a printer, is provided with a fixing device to fix the toner image on the recording medium, such as the sheet or the film. For a method of fixing the toner image on the recording medium, a heat fixing manner is generally applied to fuse a toner (a developer) by heating. To the fixing device applying the heat fixing manner, a heat roller system is often applied from a point of view of thermal efficiency and safety. Meanwhile, the heat roller system is a system which forms a fixing nip to fix the toner image on the recording medium, with a fixing roller and a pressing roller.
The heat roller system usually applies a configuration of forming the fixing roller with a hollow core bar whose material is, for example, aluminum. A halogen lamp is located inside the fixing roller to generate heat and to heat the fixing roller to a predetermined temperature. However, this system has a problem that the temperature of the fixing roller starts to rise slowly and a warm-up time becomes long.
Hence, the warm-up time may be shortened by reducing the thickness of the fixing roller and reducing a heat capacity. Then, deflection of the fixing roller becomes significant. Accompanying with this, a pressure of a center part of the fixing nip becomes weaker than pressures of both end parts of the fixing nip and fixing failure may be caused. Therefore, there is a limit to reduce the heat capacity of the fixing roller.
To solve such a problem, there is a fixing device provided with a fixing roller including a core bar and an elastic layer, a fixing belt provided around the fixing roller, and a heating source to heat the fixing belt from an outside.
By the way, in the fixing device applying such a configuration, the elastic layer of the fixing roller and the fixing belt are generally adhered by an adhesive. However, when the adhesive is used to fix the elastic layer of the fixing roller to the fixing belt, heat generated in the fixing belt escapes to an adhesive layer. Therefore, there are problems that a warm-up time becomes long and it takes time to heat the fixing belt from a standby temperature to a predetermined temperature.
Hence, there is a configuration that the fixing roller and the fixing belt are not adhered with each other and a slight gap is formed between the fixing roller and the fixing belt. There is also a configuration that an external diameter of the fixing roller becomes larger than an inner diameter of the fixing belt by thermal expansion of the fixing roller.
However, according to the configuration that the slight gap between the fixing roller and the fixing belt, when the fixing nip is formed by bringing the fixing belt and the pressing roller into pressure contact with each other, a track of the fixing belt changes from a nearly precise circular shape to an elliptical shape. As a result, when, for example, a non-contact temperature sensor is located around the fixing belt, an interval between the temperature sensor and the fixing belt changes, and a detection value of the temperature sensor becomes inaccurate. Further, when, for example, a thermostat as safety equipment is located around the fixing belt, an interval between the thermostat and the fixing belt changes, and the thermostat does not actuate at an adequate timing. For example, a so-called “early actuation” which causes the thermostat to actuate earlier than an adequate timing occurs. Further, when the fixing belt contacts peripheral members, such as the temperature sensor and the thermostat, a surface layer of the fixing belt may be damaged. Accompanying with this, it is feared that an abnormal image occurs or the fixing belt breaks. Furthermore, when an IH coil as the heating source is located outside of the fixing belt, an interval between the IH coil and the fixing belt changes and heat generation efficiency of the fixing belt is likely to decrease.
By contrast, according to the configuration that an external diameter of the fixing roller becomes larger than an inner diameter of the fixing belt by thermal expansion of the fixing roller, the fixing belt is supported from an inside by the fixing roller, so that it is possible to stabilize a track of the fixing belt.
However, the thermally expanded fixing roller comes into pressure contact with the fixing belt, and an internal pressure of the fixing belt rises and apparent hardnesses of the fixing roller and the fixing belt increase. Accompanying with this, a width of the fixing nip (hereinafter, referred to as a “nip width”) decreases. To obtain an adequate nip width despite the rise of the apparent hardnesses of the fixing roller and the fixing belt, a pressure of the fixing nip (hereinafter, referred to as a “fixing pressure”) needs to be set higher than a case where there is a gap between the fixing roller and the fixing belt (see FIG. 8). When the fixing pressure is set high, a significant load is applied to the core bar of the fixing roller. The external diameter of the core bar of the fixing roller needs to be set larger to prevent the core bar of the fixing roller from significantly deflecting even when a significant load is applied as described above. Accompanying with this, the external diameter of the elastic layer of the fixing roller also becomes large. When the diameter of the elastic layer of the fixing roller becomes large as described above, the diameter of the fixing belt also becomes large and a heat capacity and a radiation amount of the fixing belt increase, and therefore heat loss increases.
Further, as the internal pressure of the fixing belt rises as described above, a stress is constantly applied to the elastic layer of the fixing roller in use. Therefore, the fixing roller is heavily deteriorated, and a life cycle of the fixing roller needs to be set short. Deterioration of the fixing roller described herein means that, for example, the external diameter of the fixing roller decreases because a cell wall of the elastic layer of the fixing roller is crashed or the cell wall of the elastic layer of the fixing roller is torn. This phenomenon is more remarkable in a center part of the fixing roller than both end parts of the fixing roller in the rotation axis direction of the fixing roller. This is because the radiation amount of the center part of the fixing roller is lower than radiation amounts of the both end parts of the fixing roller, and thermal stress is greater. Further, the circumference of the center part of the fixing roller is trapped by the both end parts of the fixing roller, and the center part of the fixing roller deforms such that a cell is crashed and the volume of the elastic layer decreases. Furthermore, the above phenomenon is significant in a boundary surface between the core bar and the elastic layer in a radial direction of the fixing roller. This is because a stress concentrates on the boundary surface between the core bar and the elastic layer.